Prior art vacuum refining techniques which employs a refining furnace, such as a converter, connected to a pressure reducing system, have been disclosed in, for example, Japanese Laid-open Patent Application Publication Nos. 82418/82, 163549/83, 181829/83, 207311/83 and 305916/90, and Japanese Laid-open Utility model Application Publication No. 156164/85. A method of hermetically connecting a pressure reducing system to a refining furnace is mentioned specifically in, for example, Japanese Laid-open Patent Application No. 305916/90. A refining furnace mentioned in the publication comprises, in combination, a tiltable furnace body, a duct to be connected to an evacuating apparatus, and a hood having a sealing flange detachably joined to an upper part of the furnace body. The furnace body comprises a vessel provided with tuyeres in its bottom, and a conical partjoined to the vessel by a flange joint. The furnace body is provided with a sealing flange formed by expanding a part of the vessel or the conical part, and the sealing flange of the hood is seated on the sealing flange of the furnace body with an annular gasket placed therebetween to make a hermetic sealing joint between the sealing flanges.
If an amount of metal or slag which cannot be absorbed by the elastic deformation of the gasket adheres to or deposit on the sealing surface of the sealing flange of the furnace body, a reliable hermetic sealing joint cannot be made between the hood and the furnace body. It often occurs that metal and slag sputtered from within the refining surface or those once accumulated on a dust collecting hood, a furnace sealing cover or the like and fallen on the refining furnace adhere to or deposit on the furnace body. Therefore, the adhesion of metal and slag to and deposition of the same on the sealing flange must be prevented to ensure a reliable hermetic sealing joint.
As shown in FIG. 5A, a prior art refining furnace sealing method disclosed in Japanese Laid-open Patent Application Publication No. 82418/82 covers a throat flange 21 for use in hermetically sealing a furnace throat 25 formed in a refining furnace, and a taphole flange 23 for use in hermetically sealing a taphole 26 formed in the refining furnace with flange covers 22 and 24, respectively, while the refining furnace is in a state other than a vacuum refining process. The flange covers 22 and 24 prevent the adhesion of metal and slag to the flanges 21 and 23, so that a reliable hermetic sealing joint can be made for vacuum refining. The furnace throat 25 is closed by a furnace throat cover 27 and the taphole 26 is covered by a taphole cover 29 connected to an evacuating device 32 as shown in FIG. 5B upon the completion of ordinary blowing at the atmospheric pressure, and then the refining furnace is evacuated through the taphole cover 29 for degassing and refining.
Also shown in FIG. 5B are a suction pipe 28, a cooling device 30, a dust separator 31 and inert gas blowing nozzles 33.
In the refining furnace of such a construction, the flange covers 22 and 24 need to be put on the flanges 21 and 23 every time the operating mode of the refining furnace is changed from an atmospheric refining mode to a vacuum refining mode and need to be removed from the flanges 21 and 23 every time the operating mode of the refining furnace is changed from the vacuum refining mode to the atmospheric refining mode, which requires additional work using a crane or the like, and increases process time. The increase in process time reduces the productivity of the refining furnace and causes increase in wear of the refractory lining of the refining furnace due to the extension of time for which the high-temperature molten metal is held in the refining furnace. Mechanisms for holding the flange covers 22 and 24 in place are necessary to prevent the flange covers 22 and 24 from falling when tilting the refining furnace. Thus the prior art refining furnace has problems in reliability and maintainability.